1. Field of the Invention
This invention relates generally to borehole formation evaluation instrumentation and methods of using such instrumentation in the drilling of directional wells. More particularly, this invention relates to a method for measuring the position of a drillstring while drilling a horizontal borehole and maintaining the drillstring within desired boundaries using electromagnetic propagation based earth formation evaluation tools.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, tool azimuth, tool inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
In order to maximize the amount of recovered oil from such a borehole, the boreholes are commonly drilled in a substantially horizontal orientation in close proximity to the oil water contact, but still within the oil zone. U.S. Pat. No. RE35386 to Wu et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for detecting and sensing boundaries in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented. The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum. The configuration used in the Wu patent is schematically denoted in FIG. 1 by a borehole 15 having a drilling assembly 21 with a drill bit 17 for drilling the borehole. The resistivity sensor is denoted by 19 and typically comprises a transmitter and a plurality of sensors. Measurements may be made with propagation sensors that operate in the 400 kHz and higher frequency, typically 2 Mhz.
A limitation of the method and apparatus used by Wu is that resistivity sensors are responsive to oil/water contacts for relatively small distances, typically no more than 5 m; at larger distances, conventional propagation tools are not responsive to the resistivity contrast between water and oil. Wu discloses the use of a device in which a single transmitter is used and amplitude and phase measurements are made at two spaced apart receivers. U.S. Pat. No. 5,869,968 to Brooks et al. having the same assignee as the present invention discloses a dual propagation resistivity (DPR) tool in which a pair of transmitters are symmetrically disposed about a pair of receivers. With the arrangement in Brooks, it is possible to avoid the effect of mutually coupling between receivers in a propagation resistivity tool. However, even with the DPR device, it is difficult to get the necessary accuracy to see boundaries that are tens of meters from the borehole. It should be noted for the purposes of the present invention, the term boundaries includes boundaries between geologic formations as well as boundaries between different fluids in the subsurface.
An indication of the desired precision of measurements can be seen in FIGS. 2 and 3. Shown are simulations of amplitude (FIG. 2) and phase (FIG. 3) for a 3D model in which resistivity of the water-wet formation was taken as 0.2 Ωm, the resistivity of the oil-wet formation was 20 Ωm. The abscissa is the distance to the oil-water interface. Shown in FIG. 2 are amplitude ratios (in dB) for two receivers. The amplitude ratios have been normalized to amplitude ratios at a distance of 20 m, i.e., they are not absolute amplitude ratios. Similarly, FIG. 3 shows relative phase differences between measurements at the two receivers normalized to the phase difference at 20 m. The spacing between the two receivers for the model was 5 m. The spacing between the transmitter and the near receiver was 12 m.
In FIG. 2, curves 31, 32, 33, 34, 35 and 36 are the normalized amplitude ratios for frequencies of 4 kHz, 20 kHz, 60 kHz, 100 kHz, 200 kHz and 400 kHz respectively. In FIG. 3, curves 41, 42, 43, 44, 45 and 46 are the normalized phase differences for frequencies of 4 kHz, 20 kHz, 60 kHz, 100 kHz, 200 kHz and 400 kHz respectively. An important point to note is that at 400 kHz, both the amplitude ratios and the phase differences are relatively unresponsive at distances of less than 10 m. This is consistent with results shown in Wu.
The simulation results also show that even at lower frequencies, a high level of precision is required in the amplitude and phase measurements in order to use them as distance indicators. Such a precision has hitherto not been possible at lower frequency tools (less than about 400 kHz).
It would be desirable to have an apparatus and a method of using the apparatus that is able to identify bed boundaries at distances greater than 10 m for the purposes of reservoir navigation. Such an apparatus should have a high level of precision and be relatively simple to use. The present invention satisfies this need.